This invention relates to a digital-to-analog converter and to a digital communication network comprising the digital-to-analog converter.
A digital-to-analog converter is indispensable for a digital communication network. For example, the digital-to-analog converter is used in quadrature amplitude modulation to produce a quadrature amplitude modulated signal in response to a first and a second digital input signal each of which is represented by N bits, where N is an integer equal to or greater than three. In this event, the quadrature amplitude modulated signal has signal points equal to M.sup.2 on a phase plane, where M is equal to 2.sup.N.
A conventional digital-to-analog converter usually converts the digital input signal of N bits into an analog signal having 2.sup.N or M levels.
In U.S. patent application Ser. No. 779,217 filed Sept. 23, 1985 by Junichi Uchibori et al for assignment to NEC Corporation, now U.S. Pat. No. 4,675,619, a device is disclosed which circularly arranges the M.sup.2 signal points. Such a circular arrangement or distribution of signal points serves to reduce an amplitude of the analog signal, as mentioned in the above-mentioned Patent Application.
In the meanwhile, the circular distribution inevitably gives rise to occurrence of extra or additional levels which are different from the 2.sup.N levels
It is to be noted that the Uchibori et al application does not specifically teach a digital-to-analog converter which can convert such additional levels into the analog signal. In addition, an analog-to-digital (A/D) converter becomes necessary on demodulation of the quadrature amplitude-modulated signal in the digital communication network to produce the digital signals converted in the above-mentioned manner.